in usingHttpClient When initiating an HTTP request, it is possible to encounter problems with missing request headers, especially in cases like theAccept-Language Such a request header is missing. This problem can lead to incorrect content of the request and even affect the stability and functionality of the whole system. In this article, we will analyze the root cause of this problem in depth, and describe how to solve it through theHttpRequestMessage to address this issue.
1. Background of the problem: HttpClient design and sharing mechanisms
HttpClient NET is the core class for sending HTTP requests, it is a class designed to be reusable in order to improve performance and reduce the overhead of frequently creating and destroying HTTP connections in highly concurrent situations.HttpClient reuse can utilize the underlying connection pooling mechanism of the operating system, avoiding the performance loss of having to establish a new connection for each request.
But.HttpClient The mechanism of reuse may also lead to some problems, especially in the case of multi-threaded concurrent requests. For example, if we have a sharedHttpClient Frequent modifications to the request header on the instance may cause these modifications to be accidentally "passed" between requests or lost.
2. Frequently asked questions: missing request headers
Let's say we have the following code, where we want to set on each request theAccept-Language Head:
using ;
using ;
using ;
using ;
namespace ConsoleApp9
{
internal class Program
{
private static readonly JsonSerializerSettings serializerSettings = new JsonSerializerSettings
{
ContractResolver = new CamelCasePropertyNamesContractResolver(),
NullValueHandling =
};
private static readonly HttpClient httpClient = new HttpClient(); // Reusing HttpClient instances
private static readonly SemaphoreSlim semaphore = new SemaphoreSlim(100); // Limit the number of concurrent requests to 100
static async Task Main(string[] args)
{
List<Task> tasks = new List<Task>();
int taskNoCounter = 1; // For tracking taskno
// Use only one HttpClient object (globally shared)
for (int i = 0; i < 50; i++)
{
((async () =>
{
// Wait for a semaphore to control the maximum number of concurrencies
await ();
try
{
var postData = new
{
taskno = taskNoCounter++,
content = "Contents awaiting translation"
};
var json = (postData, serializerSettings);
var reqdata = new StringContent(json, Encoding.UTF8, "application/json");
// Set request header language
("Accept-Language", "en-US");
// Send request
var result = await ("http://localhost:5000/translate", reqdata);
// Read and deserialize JSON data.
var content = await ();
var jsonResponse = <Response>(content);
var response = ;
// Directly output decoded text after deserialization
($"prove：{response}");
}
catch (Exception ex)
{
($"Request Failed: {}");
}
finally
{
// Release the semaphore
();
}
}));
}
await (tasks);
}
}
// Define classes that match the response structure
public class Response
{
public int Code { get; set; }
public ResponseData Data { get; set; }
public string Msg { get; set; }
}
public class ResponseData
{
public string Content { get; set; }
public string Lang { get; set; }
public int Taskno { get; set; }
}
}
The receive code is as follows:
from flask import Flask, request, jsonify
from import translate_v2 as translate
app = Flask(__name__)
# Initialize the Google Cloud Translate client
translator = ()
@('/translate', methods=['POST'])
def translate_text():
try:
# Get JSON data from a request
data = request.get_json()
# Get the text content of the request
text = ('content')
taskno = ('taskno', 1)
# Get the Accept-Language information in the request header, default is 'zh-CN'.
accept_language = ('Accept-Language', 'zh-CN')
# Call Google Translate API for translation
result = (text, target_language=accept_language)
# Construct response data
response_data = {
"code": 200,
"msg": "OK",
"data": {
"taskno": taskno,
"content": result['translatedText'],
"lang": accept_language
}
}
# Return JSON response
return jsonify(response_data), 200
except Exception as e:
return jsonify({"code": 500, "msg": str(e)}), 500
if __name__ == "__main__":
(debug=True, host="0.0.0.0", port=5000)
Accept-Language The request header is passed through the("Accept-Language", language) to set it up. This is a common practice to specify a specific language for each request. However, in practice, especially when theHttpClient When being multiplexed to send multiple requests concurrently, this method may trigger a missing or incorrect request header.
Test result: one out of every 20 requests will receive a language that can't be picked up and will use the default zh-CN, and this request will not be translated. In the code above, the
3. Why are request headers missing?
The problem of missing request headers usually occurs in the following two situations:
-
Shared between concurrent requests
HttpClientan actual example: When multiple threads or tasks share the sameHttpClientinstances, they may modify theDefaultRequestHeadersthat cause request headers to interfere with each other across requests. For example, if a request modifies theAccept-Language, which affects all subsequent requests, rather than each request using its own request header independently. -
Header caching issues：
HttpClientInstances may cache header information. If the request headers are not set correctly, caching may result in the loss of previously set headers.
In this case, missing request headers or inconsistent request headers occur, thus affecting the correctness of the request and the accuracy of the response.
4. Solution: useHttpRequestMessage
To solve this problem, we can use theHttpRequestMessage instead of directly modifying the。HttpRequestMessage Allows us to set request headers independently for each request, thus avoiding the risk of sharing headers between multiple requests.
Here is the improved code: